Its attack on a target within Russia's recognized territorial borders was confirmed by both Moscow and by officials in Washington.

The change in U.S. policy comes at a critical time, with the Biden administration due to give way to that of Donald. Russia's Foreign Minister Sergey Lavrov has said that they will react accordingly.

Today marks a thousand days since the Russian invasion of Ukraine, and this latest development is being watched very carefully by world leaders at the G20 Summit in Brazil.

More on that in a moment, but first our Russia editor Steve Rosenberg has this report from Moscow. Breaking news on Russian TV.

Claims that Ukraine had fired six American Atakams missiles into Russia's Bryansk region, though no reports of casualties.

The response from Russia's Foreign Minister Sergey Lavrov said the missile attack was a signal that the West wants to escalate the conflict.

But is this escalation by Russia?

Published today, a Kremlin decree lowering the nuclear threshold.

Russia now reserving the right to go nuclear even if attacked by conventional weapons, if such an attack threatens Russia's sovereignty or territorial integrity.

President Putin had promised changes to the nuclear doctrine weeks ago.

A clear signal to Europe and America not to let Ukraine strike Russia with long-range missiles.

Then an even clearer signal.

Russian nuclear exercises are warning to the West not to cross Russia's red lines.

But the key question in Russia's war on Ukraine, would Vladimir Putin use a nuclear weapon?

I think it's quite serious because even Putin doesn't know whether he can use nukes or he can't.

It depends on his emotions.

We know that he's a very emotional man and the decision to begin this war was also an emotional step.

The lowering of the nuclear threshold does not mean that Vladimir Putin is about to press the nuclear button.

These changes are designed, I think, at the very least, to put pressure on Western leaders who will now have to decide, is Putin bluffing or not? Earlier, at the Museum of Victory, the Kremlin kick-started commemorations for next year's 80th anniversary of the end of World War II.

Not a word was said here about the 1,000 days of Russia's war in Ukraine.

A war which, for the Kremlin, hasn't gone at all to plan, but which it remains determined to win.

Steve Rosenberg, BBC News, Moscow. So what is the situation on the ground in Ukraine in terms of territory gained by Russia?

I'm joined by BBC Verify's Nick Erdley. Nick.

Thank you, Rita. 1,000 days of war, I've seen a number of ups and downs for Ukraine, but analysis of the picture since the start of this year shows things are going far better for Russia.

Let me show you the picture for 2024, based on analysis by the Institute for the Study of War.

This is the picture in February.

If we fast-forward to September and look at this area here, you can see Russia has made inroads.

This might look like a small amount on the map, but it's a significant amount of territory on the ground.

And that advance has continued in Russia has seized nearly 2,000 square kilometers of territory, just over 1,000 square miles.

That's nearly six times more than it did in the whole of 2023. Let me show you what all of this means in practice.

This is Kurokov in eastern Ukraine.

You can see it on the map there.

And if we show you this video, we can see the devastation.

This is a key town Russia is trying to capture.

It's come under heavy bombardment recently as part of the Russian offensive.

You can see the damage that that's caused.

There's been heavy fighting in the Kursk region of Russia, too.

Ukrainian troops captured, you can see it here, a large amount of territory in August.

But Russian forces have now retaken, as you can see here, around half of that.

All of these gains have come at a cost for Russia.

Tens of thousands of troops have died.

But as Moscow is willing to pay.

And this is important with President Trump's return to the White House looming.

Mr. Trump has said he can end the war in a day.

It's not clear how.

But if there were peace talks, territory will be an important bargaining chip.

And some analysts think that Russia's recent gains will give it a stronger hand in any potential discussions.

Rita. Nick, thank you very much. Well, the G20 world leaders are meeting in Brazil, as you heard.

And President Zelensky of Ukraine had some bitter words for them, making clear his frustration tonight at their lack of response to President Putin's change of Russia's nuclear doctrine.

Well, in a moment, we'll speak to our chief international correspondent, Lisa Doucette.

But first to Vicky Young, who is in Rio for us.

And Vicky, what have leaders there been saying? Well, Rita, there's a pretty wide gulf here between the G20 countries in their approach to Ukraine, particularly how critical to be of Russia.

So that joint declaration that they put out at the end of these summits, well, the language in that is weaker than the same time last year at the equivalent summit.

So you can understand maybe why President Zelensky is pretty frustrated by that. When it comes to the British Prime Minister, he said that at every opportunity during this summit, he has raised the issue of Ukraine with the other leaders, saying that they really should have what they need for as long as they need in order to win this war.

Now, whether that means that British-made missiles will end up being used in the same way as the American ones have been, well, we don't know.

He just won't talk about that.

He says that's operational, and all it would do is help the Russians.

He was also asked, though, about those comments from Russia about how and when they might use nuclear weapons.

There's irresponsible rhetoric coming from Russia, and that is not going to deter our support for Ukraine.

We're now on day 1,000 of that conflict.

That's 1,000 days of Russian aggression, 1,000 days of sacrifice in Ukraine.

Now, Kistama, they're putting the blame firmly on President Putin, on Russia, saying they are the aggressor and saying they could just stop this war now by stopping that illegal invasion.

Now, earlier, Downing Street actually called the Russian government depraved.

So you can hear there that the rhetoric is really escalating.

The fear amongst many leaders across the world is that the situation on the battlefield will escalate, too.

Vicky, thank you very much.

Vicky Young there in Rio de Janeiro.

Well, Lise, we've seen Nick show us the fast land grab by Russia.

Steve earlier talked about the nuclear threat.

What does all this add up to? Well, Rita, tonight, understandably, a lot of the focus is on this nuclear threat, and it's not reassuring to hear the Russian columnist in Steve's piece frame it as an this risk.

But President Putin has many other ways to ratchet up the cost for the West and is already using them.

As you've been hearing this week, the sabotage of undersea cables by Germany, then Poland is talking about the planting of bombs on planes, and so on.

But the biggest question, of course, is what cost, what greater cost is Russia going to impose on Ukraine?

As we've also been hearing, Ukraine is very much on the back foot, on the front lines, and is hoping that striking deeper inside Russia is going to strengthen its position in this war and in any negotiating tables which seem likely to emerge.

But it may not work, and especially when President-elect Donald Trump moves into the White House with his threat to end the war.

And so the big question now is what happens when President Trump takes power?

Will he reverse this decision regarding the That must be on President Putin's mind as well.

It's on many minds.

So you have two unpredictable presidents at this moment of mounting risk.